The present invention relates to a process for detecting a misfire in an internal combustion engine, and in particular a process which can be used for detecting a misfire in one or more cylinders of an internal combustion engine. The present invention also relates to a system for carrying out said process.
It is known that in order to monitor the performance of an internal combustion engine, in particular a racing engine with a high number of cylinders, it is desirable to detect the occurrence of the misfire of the fuel mixture in one or more cylinders. A process for carrying out said detection, which is known from U.S. Pat. No. 5,576,936 and presently plays an important role with respect to the ever stricter rules for the control of polluting exhausts, consists of measuring the sudden fluctuations in the rotational speed of the crankshaft by means of an electronic sensor located close to the fly-wheel. This sensor is connected to the control unit positioned inside the car, which receives all the data concerning the engine and transmitted by suitable sensors. By calculating the fluctuations in the speed according to the delivered torque it is possible to identify a possible misfire in one cylinder of the engine. However, this process does not allow to precisely identify in which cylinder the misfire occurred and moreover has a quite high error probability, particularly in the case of the traveling car being subjected to sharp oscillations, e.g. caused by defects in the road surface, which temporarily affect the rotational speed of the crankshaft.
In order to overcome these drawbacks, U.S. Pat. No. 5,109,825 devised to measure the fluctuations in time of the pressure of the engine exhaust gas. Though pressure sensors available on the market are very accurate and provide a response almost in real time, the known processes for detecting the misfire on the basis of the measurement of the pressure fluctuations in the exhaust gas are still very inaccurate and poorly reliable, particularly when applied to engines with a high number of cylinders.
Therefore the object of the present invention is to provide a process for detecting the misfire which is free from the above-mentioned drawbacks. Another object of the present invention is to provide a system which carries out said process. These objects are achieved by means of a process and a system whose main characteristics are disclosed in the first and eighth claim respectively.
Thanks to the sampling and the subsequent frequency analysis of the pressure values detected in the exhaust pipes, the process according to the present invention provides a higher accuracy and reliability with respect to prior art processes. In fact, if the engine firing is regular, the periodical openings of the cylinder exhaust valves generate pressure pulses in the exhaust pipes having the same periodicity and similar waveforms. On the contrary, in the case of misfire in one of the cylinders, the corresponding pressure pulse is changed, thus changing the periodical pattern of the pressure values. The reference for the synchronization with the pulse frequency is readily derivable from the sensors detecting the rotational speed of the crankshaft and/or camshaft.
Another advantage of the process according to the present invention is that through the frequency analysis of the sampled signal it is possible to determine whether only one or more misfires occurred during a single engine cycle. In fact, the amplitude of the modulus of the various harmonics of the sampled signal depends on the number of cylinders wherein the misfire occurred.
A further advantage of the process according to the present invention is that through the frequency analysis of the sampled signal it is possible to determine not only the misfire, but also the position of the cylinder where it occurred. In fact, the knowledge of the cylinder firing sequence and the comparison of the phase of the first harmonic of the sampled signal with the phase of the first cylinder provide a phase difference which indicates the position of the cylinder where the misfire occurred.